(1) Field of Art
This invention relates to a novel solderable alloy of nickel and indium which is useful in the manufacture of electrical devices, and to an electroplating bath for use in the manufacture of such alloy. Yet another aspect of this invention relates to printed circuit boards in which the nickel/indium alloy of this invention is used as the etch resist, and as a protective coating over the copper circuits. Yet another aspect of this invention relates to a printed circuit board and other electrical devices in which the circuitry is composed of a nickel/indium alloy. Still another aspect of this invention relates to a method of preparing such printed circuit boards and printed circuit boards prepared by such method. Other aspects of this invention will become apparent from a purview of the specification and appended claims.
(2) The Prior Art
Printed circuit boards are widely used by the modern electronics industry as interconnecting devices. Prior art manufacturing processes for manufacturing such boards are characterized as subtractive, additive, or semi-additive.
The general technique for forming printed circuit boards by means of subtractive processing is well known. Examples of the subtractive process are described in detail in U.S. Pat. Nos. 3,673,680 and 4,135,988. In one modification of the subtractive process, a copper laminated substrate (after an appropriate preparatory step, as for example cleaning) is covered with a layer of protective photoresist. The photoresist layer is subsequently masked with a desired circuit pattern, and exposed to a source of energy, such as a source of ultraviolet radiation. Where struck by the ultraviolet radiation, the photoresist is polymerized and made insoluble in the developer. The preselected circuit pattern or image areas are revealed when unexposed portions of the photoresist are washed away by developer. Additional copper is then deposited on the bare copper which had been covered by the unexposed portion of the photoresist through use of standard electroplating techniques, followed by electrodeposition of a 60/40 tin/lead alloy etch resist in a thickness of from about 10 to about 15 microns also using such standard techniques. The remaining photoresist is then removed. The surface of the copper clad laminate now consist of areas of bare copper which are etched away through use of a standard etchant, as for example an alkaline etchant such as ammoniacal copper, and acidic etchants such as hydrogen peroxide in sulfuric acid, and areas of copper which are protected from the etchant by the productive metallic etch resist. After etching, the tin/lead alloy covering the copper circuits is then reflowed to provide bright homogenous circuits suitable for soldering. After soldering, residues of flux can be removed.
In addition, the printed circuit can be drilled to provide holes therein, drilling being carried out at any convenient stage in the process, as for example after the initial cleaning of the copper laminate. If the printed circuit is to be double-sided with plated through holes which establish electrical contact between a copper circuit one side and a copper circuit on the other side, then the inside surfaces of the drilled holes are cleaned and coated with catalyst, e.g., a palladium/tin catalyst, and a thin layer of copper or other suitable electrically conductive material is then deposited on the inside surfaces of the holes by an electroless plating techniques. Further, additional electrically conductive material can be electroplated onto the first layer of electroplated material to provide a thickness of electrically conductive material sufficient to carry the required electrical current. If components are to be mounted on the printed circuit board using a flowing solder technique, e.g., using a wave soldering machine, then a solder mask is printed over those parts of the copper circuit which are not to receive solder. As an alternative to electroplating a tin/lead alloy over the hole of the copper circuit the tin/lead alloy can be selectively plated only onto those parts of the circuit which are to receive solder. This alternative technique involves printing a different etch resist on those parts of the circuit not covered by the tin/lead alloy which leads to registration and other problems.
In the semi-additive process for manufacture of printed circuit boards, through-holes are drilled in an insulating substrate, followed by activation of the substrate and through-holes with an activating solution typically containing noble metal ions. This is then followed by the deposition of electroless copper on top of the substrate and on the walls of the formed holes, accomplished by immersion in an electroless copper plating solution. Thereafter, a first resist, such as a photoresist or screen printable resist is applied to the formed board in the negative image of the desired circuit pattern leaving exposed those regions of the electroless copper corresponding to the desired circuit pattern. A copper layer is then electroplated on top of the exposed electroless copper portions, including the hole walls. The first resist is removed and the structure is immersed for a time in a copper etchant which removes the uncovered electroless copper. Alternatively, the electroplated surfaces of the substrate can be electroplated with a second etch resist such as silver, tin, lead or gold to cover the surfaces of the electroplated copper prior to removal of the first resist followed by removal of the first resist and etching of the copper. The copper underlying the second etch resist forms the desired circuit on the circuit board.
These known substractive and semi-additive processes of producing printed circuits, and particularly printed circuits with plated through holes are relatively complex and involve a large number of different steps, require energy for the removal of the tin/lead alloy from those portions of the copper circuit in the edge contact area and for the reflowing of the tin/lead alloy, and are relatively expensive. Moreover, the printed circuits produced by these known methods are unsatisfactory in that during flow soldering of components to the printed circuit the tin/lead alloy, which has a low melting point of 190.degree. C., tends to run under the solder mask under the heat of the soldering operation (usually about 230.degree. C.) potentially causing a short circuit and maring the appearance of the printed circuit with the components mounted thereon.